Optical switching device

ABSTRACT

An integrated optical switching device for switchably converting a fraction of a first guided mode signal into a second guided mode signal of different order is provided. Added to a passive mode converter (c) provided with a bimodal waveguide, in which the conversion can take place by a periodic coupling in coupling surfaces 1-N as a consequence of a specific geometry (f, g), are electrodes (10, 14) for switchably disrupting the coupling, as a consequence of which the conversion does or does not take place. Preferably, the optical switching device is constructed on semiconductor material and the modification is carried out by charge carrier injection. On/off and directional switches based on this are provided. As a result, the advantages of the present system are very good integrability, short length, no critical parameters in the manufacture, and operation at low control currents.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of integrated optical devices. Moreparticularly it relates to an optical switching device for switching alight signal.

2. Prior Art

In a coherent optical local network, a subscriber is connected by meansof a combined coherent optical transmitter/receiver. Such a combinedtransmitter/receiver is preferably constructed as an integrated device.In such a transmitter/receiver, a switch should be incorporated forclosing the transmission channel in order to be able to adjust thetransmitter without disrupting the network.

For such a switch, an isolation or extinction ratio is required of atleast approximately 40 dB. Various types of switches, 1×2 or 2×2, areknown, such as the directional coupler, the digital optical switch, or aMach-Zehnder switch. These known switches generally have an isolation of20 to 30 dB and do not therefore achieve the required isolation. It istrue that the required isolation can be achieved by placing two or moresuch switches in series. This means, however, that the complexity andthe integration space needed increases. Another switching possibility,which directly fits in with the use of semiconductor material, can beobtained by means of charge carrier injection. As a result of injectingfree charge carriers into the semiconductor material over a certainlength of a waveguide via an electrode, the absorption increases. Inprinciple, any desired isolation could be obtained simply by making saidlength and/or the current of charge carriers large enough. An absorptionswitch of this type would, in addition, be very simple since it can beformed by a waveguide and an electrode placed lengthwise thereon. Withthe present state of the art of making contact, however, semiconductormaterials, such as InP, do not withstand current densities greater than20 kA/cm², which is equivalent to a length of not less than 1 cm for thedesired 40 dB isolation. In view of the fact that the typical dimensionsof a `chip` of semiconductor material are at present between 1 and 2 cm,this is long.

Reference [1] discloses a directional coupler on semiconductor material,which directional coupler makes use, for the control thereof, of chargecarrier injection over the length of the coupling section with the aidof an electrode placed centrally above the central coupling section. Inthis switch, too, the isolation or extension ratio is of the order of 20dB.

In a bimodal waveguide, light signals can generally propagate, within aparticular wavelength range, both in a zeroth-order guided mode and in afirst-order guided mode. If said bimodal waveguide merges into amonomodal waveguide via a taper, only the zeroth-order mode component ofthe signal in the bimodal waveguide propagates further in the latter andthe first-order guided mode component is scattered in the taper. If thebimodal waveguide debouches in an asymmetrical Y junction having abimodal input guide and two monomodal output guides with differentpropagation constants, the two mode components are split, specificallyin such a way that the zeroth-order guided mode component will coupleout via the output guide having the highest propagation constant and thefirst-order guided mode component via the one having the lowestpropagation constant and will propagate further therein in thezeroth-order mode of said guide. In both cases, the two signalcomponents are therefore separated from one another in this process, oneof the components being lost, however, in one case. Reference [2], whichhad not been laid open for inspection to the public in time, describes apassive integrated optical device which can be dimensioned in such a waythat a well-defined signal fraction, up to 100%, of a zeroth-orderguided mode is converted therewith in a wavelength-selective manner intoa first-order guided mode in a bimodal waveguide. If such a converter iscoupled at its output to a taper or an asymmetrical Y junction asindicated above and if the conversion of the signal fraction were alsoto be switchable between two states in which conversion does or does nottake place, respectively, a switch would be produced for switching thesignal fraction. If the taper is chosen as output section, an on/offswitch is produced, while an asymmetrical Y junction produces apropagation direction switch. In view of the constituent components ofsuch a switch, it may be expected that it can overcome the drawbacksmentioned above. In addition, a wavelength-selective multiplexer anddemultiplexer described in reference [3], which had not been laid openfor inspection to the public in time, could be of switchableconstruction. There is therefore a need for an optical device of a typesimilar to the mode converter which is described perse in reference [2]and in which the conversion is switchable.

SUMMARY OF THE INVENTION

The object of the invention is to provide for the need mentioned. Forthis purpose, an optical switching device is, according to theinvention, characterised by a passive mode converter for converting afraction of a first guided mode signal into a second guided mode signalby means of a periodic coupling between the first and second guided modesignals, the mode converter including a waveguide having in itslongitudinal direction a periodic mode field structure, and means forswitchably modifying the periodic mode field profile structure in thewaveguide, which is constructed in an optical medium to which the meanscorrespond for allowing switchably modifying. The invention is based onthe insight that, in a channel-type waveguide, and more in particularlyin a bimodal channel-type waveguide, the mode field profiles of azeroth-order guided mode signal and a first-order guided mode signal arecharacteristically different and, on the basis of this, one of theprofiles can be more strongly modified than the other.

In addition, an object of the invention is to provide an optical switchof the type indicated above which does have the required insulation butdoes not have the disadvantages mentioned.

It is pointed out that switchability also implies modulability, with theresult that the various switching devices according to the invention canalways also be used as intensity modulators.

The invention produces a switching device which is very readilyintegrable, preferably on semiconductor material such as InP, and whichis much shorter, approximately 1 mm, but hardly any more complicatedthan an absorption switch of the type indicated above. The waveguidestructure can be produced in a single etching step. There are nocritical parameters in the manufacture. It requires control currentswhich are much lower than usual for absorption switches.

REFERENCES

[1] K. Ishida, et al. "InGaAsP/InP optical switches using carrierinduced refractive index change", Appl. Phys. Lett. 50(3), 19 Jan. 1987,pp. 141, 142;

[2] EP-A-o513919 (by the Applicant; published Nov. 19, 1992) entitled:Mode converter;

[3] NL-9101532 (by the Applicant; not yet published) entitled:Golflengte selectieve multiplexer en demultiplexer [Wavelength-selectivemultiplexer and demultiplexer].

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail by means of thedescription of a number of exemplary embodiments, reference being madeto the drawings wherein:

FIG. 1 shows diagrammatically an optical switching device according tothe invention in a plan view;

FIG. 2 shows a cross section of the switching device shown in FIG. 1;

FIG. 3 shows, for a specific switching device as shown in FIG. 1, thedegree of conversion as a function of a phase deviation between theguided modes to be coupled in consecutive coupling surfaces of a modeconverter used in the switching device.

DESCRIPTION OF PREFERRED EMBODIMENTS

Mode field profile of a light signal propagating in a channel-typewaveguide is understood as meaning (the shape of) the electrical fielddistribution which a guided mode of the light signal has in thewaveguide. Said profile depends not only on the geometry of the guide,called geometric structure, but also on the optical properties of thewaveguide medium and its surroundings, called refractive index profile.The total structure of a waveguide, i.e. its geometrical structure andits refractive index profile, is called mode field profile structure. Ina, e.g. bimodal, channel-type (optical) waveguide in a medium which isoptionally isotropic, two propagation modes, viz. the fundamental orzeroth-order mode and the first-order mode, of each polarization aregenerally able to propagate. The mode field profiles which such abimodal waveguide has for the two guided modes have a characteristicdifference. Specifically, in the center of such a channel-type guide,the field strength associated with the first-order propagation mode iszero, but that of the zeroth-order propagation mode is, on the otherhand, at a maximum. With a suitably chosen medium for the waveguide,this provides the possibility of modifying essentially only thepropagation constant of one of the two guided modes in the waveguide,that is to say of the zeroth-order or the first-order mode, andconsequently modifying the mode field profile of the bimodal guide forthat guided mode, and not modifying, or modifying to a much smallerextent, that of, or for, the other guided mode. Since a geometricalstructure, once chosen, of a waveguide has essentially been established,modifying the mode field profile should preferably occur by modifying ofthe refractive index profile. The invention applies this to achannel-type guide of a per se passive mode converter of types such asthose described by reference [2], and more particularly indicated inTABLE 2 therein. Such a mode converter contains a central waveguide,which has a mode field profile structure, which varies periodically inits longitudinal direction. As a consequence in the waveguide a numberof coupling surfaces are present on a mutual distance of half a period,the distance of a full period being called period length PL, whereby inthe waveguide a periodic coupling is effected between the two guidedmodes, for which the converter has been dimensioned. With each pair ofguided modes a specific propagation distance corresponds, calledcoupling length KL, over which coupling distance the two modes involvedcome into phase again. Should in the central waveguide of the passivemode converter a positive interfering coupling take place for convertinga certain fraction of the one guided mode into the other guided mode,then the coupling length KL of the pair of guided modes involved shouldmatch with the period length PL. Modifying the mode field profile ofmainly one of both guided modes in a manner as described above resultsin the coupling length KL between both the modes involved in theconversion being modified and therefore not matching necessarily anymore with the period length PL. In that case the modes involvedexperience a mutual phase deviation in each coupling surface, as aconsequence of which an insufficiently positive interfering coupling isestablished. The conversion then no longer completely takes place to theextent to which the coupling length and the number of coupling surfacesin the passive converter are matched, or it does not take place at all,all this being dependent on the extent of the modification. If themodification is switchable between two states, for example nomodification and, on the other hand, modification, the conversion of acertain fraction of the one mode into the other is thereby switchable.The reverse effect is also possible. A particular mode converter may, asit were, be designed as critically detuned for the conversion of aparticular fraction of one guided mode into another, with the resultthat the desired conversion only takes place with a suitably chosendegree of modification, while said conversion does not, on the otherhand, take place without the modification. Just as for the mentionedpassive mode converter, it is also true of such switchable modeconverters that they can be produced using known integrationtechnologies throughout, and on the basis of waveguide materials andstructures used in such technologies. The waveguide materials are,however, subject in this connection to the limitation that they musthave the possibility of the modification of the mode field profilestructure indicated above, i.e. of the refractive index profile of thewaveguide. Theoretically, the desired modification of the lightpropagation in the bimodal waveguide can be achieved by means ofelectro-optical, thermo-optical, opto-optical etc. effects, givensuitable choice of the material of the waveguide or of its surroundingsand the associated modifying means. Since the modification has to takeplace fairly directionally, use is preferably made, however, ofelectro-optical effects, and more particularly, of charge carrierinjection into the semiconductor materials.

A structure of a switching device based on a switchable mode converterwill be described in more detail below only by way of example on thebasis of one type of waveguide material, viz. InP, and one type ofwaveguide, viz. a ridge-type waveguide, with matching modifying means.

FIG. 1 shows diagrammatically a plan view of an optical switching deviceaccording to the invention, while FIG. 2 shows diagrammatically a crosssection of such a device. The device comprises, adjoining one anotherconsecutively, a monomodal incoming waveguide a, a wave-guiding taper bfrom a monomodal to a bimodal waveguide, a 100% TX₀₀ →TY₀₁ modeconverter c, a second wave-guiding taper d from a bimodal to a monomodalwaveguide, and a monomodal outgoing waveguide e. The mode converter chas a central bimodal waveguide f in which, by means of constrictionshaving a length L which are also repeated periodically over a length L,a number of coupling surfaces, numbered 1, 2, --, N, is provided foreffecting a wavelength-selective periodic coupling between the guidedmodes TX₀₀ and TY₀₁. Here both TX and TY stand for one of thepolarizations TE or TM. For the sake of simplicity only, theconstrictions have been chosen here as symmetrical and with the samelength as the wider parts of the waveguide f situated between theconstrictions g. Situated centrally above the bimodal waveguide f is anarrow elongated strip-type electrode 10 which preferably extends overall the coupling surfaces 1 to N inclusive, that is to say in the lengthdirection of the waveguide f viewed from upstream of the first couplingsurface 1 to past the last coupling surface N.

The entire waveguide structure from the incoming guide a to the outgoingguide e inclusive is of the type of a ridge-type waveguide, whose crosssection is shown in FIG. 2. Situated between a substrate 11 and an upperlayer 12, both made of InP, is a light-guiding layer 13 made of InGaAsPand having thickness t. The upper layer 12 has, locally over the entirelength of the waveguide structure, a ridge-type elevation 12.1 having afixed height h with a total height H, and having a width w which isdifferent for the various adjoining waveguides a to e. Situatedcentrally on the ridge-type elevation of the bimodal waveguide f of theconverter c is the strip-type electrode 10, while a laminar electrode 14extends over the bottom of the substrate 11, at least underneath thebimodal waveguide f. A switchable current source can be connected to theelectrodes 10 and 14 via supply and collecting conductors (not shown) inorder to supply a current, by means of which charge carrier injectioncan be effected, as known, into the upper layer 12 at the position ofthe ridge-type elevation 12.1, as a consequence of which a change in therefractive index is effected. The operation of the switching device isas follows. With the current source switched off, no charge carrierinjection takes place and the mode converter c therefore behaves as apassive device. If a zeroth-order guided mode light signal having apolarization TX and a wavelength for which the mode converter c isselective enters the bimodal waveguide f via the incoming waveguide aand the taper b under these conditions, said signal is completelyconverted in the converter into a TY₀₁ signal. Having arrived in thetaper d, where the channel-type waveguide is narrowed down from abimodal to a monomodal guide, said first-order guided mode signal isunable to propagate further in the guide but is scattered therein. Ifthe current source is now switched on, charge carrier injection takesplace into the light-guiding layer 12 underneath the electrode 10, moreparticularly into the ridge-type elevation 12.1 of the bimodal waveguidef. As a result, the fixed coupling length L provided geometrically bymeans of the periodic constrictions is no longer capable of effecting apositive interference in the consecutive coupling surfaces, as isnecessary for mode conversion. In each coupling surface, the modes to becoupled get more and more out of phase. As the degree of charge carrierinjection increases, an ever increasing "detuning" takes place, with theresult that eventually conversion no longer takes place. The signalwhich has entered via the incoming guide a will now in fact pass throughthe switching device unaltered and leave via the outgoing guide e. Anyother guided mode signals, likewise incoming via the incoming waveguidea and for which the mode converter c is not selective, will again leavethe switching device via the outgoing waveguide e, in principleunaltered in both cases.

Instead of being a single strip-type electrode 10 which is situatedcentrally on the ridge-type elevation 12.1 and with which the mode fieldprofile of the zeroth-order guided mode is essentially modified, theelectrode 10 may also be of twin construction, with two coupled strips,which are situated essentially symmetrically with respect to the centerand, preferably, near the edges of the ridge-type elevation 12.1. Insuch a design, essentially the mode field profile of the first-orderguided mode is modified on energizing the electrodes.

The actual switchable mode converter according to the invention isformed by the mode converter c in combination with the matchingmodifying means, in this case the electrodes 10 and 14, to which aswitchable current source can be connected.

EXAMPLE

The following values illustrate an on/off switch for a TE₀₀ signal for awavelength of 1.5 μm with a structure as described by reference to FIGS.1 and 2:

for the ridge-type waveguide structure

refractive index of InP, n₁ =3.1753,

refractive index of InGaAsP, n₂ =3.4116

t=0.473 μm, H=0.504 μm and h=0.2 μm.

This ridge-type waveguide structure can be produced in one etching stepthe width w being the sole variable parameter:

the incoming waveguide a must be monomodal for TE; therefore w=6.0 μmmaximum;

the taper b must run from monomodal to bimodal, the bimodal channelhaving good guiding properties at least for the TM polarization; forthis purpose, a width of w=8.5 μm is suitable, while approximately 1° ischosen as taper angle for the transition; the length of the taper isconsequently approximately 200 μm;

a 100% TE₀₀ →TM₀₁ with N=11 coupling surfaces and a coupling length L=65μm is chosen as mode converter c; in each constriction g, the width w=6μm; the total length of the mode converter is approximately 710 μm;

the strip-type electrode 10 has a width of 3 μm; the remainingdimensions and the choice of material for the electrodes are in factirrelevant for the operation of the device; usually a thickness ischosen of approximately 200 nm, with a layered structure of Ti (2-5 nm),Pt (2-5 nm) and Au;

for the second taper d from bimodal to monomodal, the chosen dimensionsare equal to those of the first taper b;

the outgoing waveguide e must be monomodal for the polarization TM; thisis achieved with a width of not more than 4.3 μm. The total length ofthis is somewhat more than 1 mm, which is appreciably shorter than thatachievable with the known absorption switch.

In FIG. 3, the degree of conversion M (vertically from 0 to 100%) isplotted for the switching device of the example as a function of the"detuning", i.e. the phase deviation Φ between the modes to be coupledin each coupling surface (horizontally in rad). For Φ=0 rad, M=100%,that is to say complete conversion. For Φ=0.5 rad, M≈0.01% (arrow A),that is to say the converted fraction is virtually zero. The currentdensity associated with such a phase deviation is only 3 kA/cm², whichis equivalent to a very low injection current of 65 mA. The associatedattenuation is <0.5 dB. Something similar is the case for Φ=1.1 rad(arrow B), a subsequent minimum in the degree of conversion, albeit at asomewhat higher injection current. An on/off switching of the conversionis therefore obtained in this example if the injection current is alwaysset in such a way that, for a first current value, the phase deviationis Φ=0 rad, that is to say the conversion is 100%, and for a secondcurrent value, the phase deviation Φ=0.5 rad, and the conversion istherefore virtually zero. From the figure it is evident that virtuallyany degree of conversion between 0 and 100% can be produced bycontinuously regulating the phase deviation between 0 and 0.5 rad whilecontinuously regulating the injection current by energizing theelectrodes 10 and 14. Such devices are therefore also suitable foranalog signal modulation.

In the example, the passive mode converter used in the switching deviceis designed for a specific selective conversion, which means that, ifthere is no injection current, there is, in principle, no phasedeviation (Φ≈0) for the mode for which the mode converter is selective.However, a mode converter can also be manufactured with a certain fixedphase deviation (Φ≠0) in the absence of an injection current, that is tosay, as it were, critically detuned, it being possible to obtain thedesired degree of conversion (0-100%) by regulating the injectioncurrent.

A switching device described by reference to FIG. 1 is in fact an on/offswitch for that fraction of a zeroth-order guided mode signal for whichthe mode converter is selective. If the taper d at the output side ofthe device is replaced by an asymmetrical Y junction h based on amonomodal branching of a bimodal waveguide, that is to say having twooutgoing monomodal branches with different propagation constants, apropagation direction switch is obtained which is specific for thatfraction for which the mode converter is selective. Such a 2×2 switch isproduced if the taper b is also replaced at the input side by such a Yjunction k.

A polarization-independent switching device can be produced if twoswitchable mode converters which are selective for differentpolarizations, whether or not separately switchable, are placed inseries between tapers and/or asymmetric Y junctions.

I claim:
 1. Optical switching device, comprising:a passive modeconverter, provided with a bimodal channel type optical waveguide havinga periodic mode field profile structure with a fixed period forconverting a fraction of a first guided mode signal into a second guidedmode signal, one guided mode of the first and second guided mode signalsbeing a guided mode of a zeroth-order and another guided mode of thefirst and second guided mode signals being of a first-order, andswitching means for switchably modifying mainly a propagation constantof only one of said first and second guided mode signals in the channeltype optical waveguide.
 2. Optical switching device according to claim1, wherein:the bimodal waveguide is constructed as a channel-typewaveguide in semiconductor material, and said switching means comprise afirst electrode and a second electrode for effecting charge carrierinjection into the semiconductor material of said channel-typewaveguide, the first electrode being of a strip-type and extending inthe longitudinal direction above said channel-type waveguide.
 3. Opticalswitching device according to claim 2, wherein the first electrodecomprises a strip-type element centrally situated in the longitudinaldirection above said channel-type waveguide.
 4. Optical switching deviceaccording to claim 3, further comprising:a first wave-guiding transitionsection from a first incoming monomodal waveguide to the bimodalwaveguide of the mode converter, and a second wave-guiding transitionsection from the bimodal waveguide of the mode converter to a firstoutgoing monomodal waveguide.
 5. Optical switching device according toclaim 3, wherein said mode converter is a 100% converter.
 6. Opticalswitching device according to claim 5, further comprising:a firstwave-guiding transition section from a first incoming monomodalwaveguide to the bimodal waveguide of the mode converter, and a secondwave-guiding transition section from the bimodal waveguide of the modeconverter to a first outgoing monomodal waveguide.
 7. Optical switchingdevice according to claim 2, wherein the first electrode comprises twostrip-type elements situated in the longitudinal direction essentiallysymmetrically with respect to a center of, and above, said channel-typewaveguide.
 8. Optical switching device according to claim 7, furthercomprising:a first wave-guiding transition section from a first incomingmonomodal waveguide to the bimodal waveguide of the mode converter, anda second wave-guiding transition section from the bimodal waveguide ofthe mode converter to a first outgoing monomodal waveguide.
 9. Opticalswitching device according to claim 7, wherein said mode converter is a100% converter.
 10. Optical switching device according to claim 9,further comprising:a first wave-guiding transition section from a firstincoming monomodal waveguide to the bimodal waveguide of the modeconverter, and a second wave-guiding transition section from the bimodalwaveguide of the mode converter to a first outgoing monomodal waveguide.11. Optical switching device according to claim 1, further comprising:afirst wave-guiding transition section from a first incoming monomodalwaveguide to the bimodal waveguide of the mode converter, and a secondwave-guiding transition section from the bimodal waveguide of the modeconverter to a first outgoing monomodal waveguide.
 12. Optical switchingdevice according to claim 11, wherein the first transition section andsecond transition section are single wave-guiding tapers.
 13. Opticalswitching device according to claim 11, wherein the first and secondtransition sections are asymmetrical Y junctions.
 14. Optical switchingdevice according to claim 11, wherein the first transition section is asingle wave-guiding taper and the second transition section is anasymmetrical Y junction.
 15. Optical switching device according to claim2, wherein said mode converter is a 100% converter.
 16. Opticalswitching device according to claim 15, further comprising:a firstwave-guiding transition section from a first incoming monomodalwaveguide to the bimodal waveguide of the mode converter, and a secondwave-guiding transition section from the bimodal waveguide of the modeconverter to a first outgoing monomodal waveguide.
 17. Optical switchingdevice according to claim 2, further comprising:a first wave-guidingtransition section from a first incoming monomodal waveguide to thebimodal waveguide of the mode converter, and a second wave-guidingtransition section from the bimodal waveguide of the mode converter to afirst outgoing monomodal waveguide.
 18. Optical switching deviceaccording to claim 1, wherein said mode converter is a 100% converter.19. Optical switching device according to claim 18, further comprising:afirst wave-guiding transition section from a first incoming monomodalwaveguide to the bimodal waveguide of the mode converter, and a secondwave-guiding transition section from the bimodal waveguide of the modeconverter to a first outgoing monomodal waveguide.